This invention relates to a low-temperature process for preparing thin (e.g., 0.1-100 .mu.m) glass films which can be applied to metal, glass, or ceramic substrates by simple commercial processes such as spraying, dipping or spinning.
Currently, there is much interest in the replacement of organic-polymer-based coatings with glass coatings for use in hostile environments, e.g., high temperature, corrosive, or abrasive environments. The requirements of such a coating include: (1) that it behave like a glass (i.e., be refractory, corrosion resistant, electrically insulating and fully dense and cohesive with no pin holes); (2) that it be sufficiently flexible, for example, to be coated on wire and wound on a coil; and (3) that it be applied at low temperatures compatible with the desired substrate.
The most common methods of applying glass films to metal, glass and ceramic substrates are: (1) enameling or glazing; (2) chemical vapor deposition; and (3) direct immersion in a molten glass. These methods all have significant disadvantages with regard to the preparation of thin, flexible films.
Glazing or enameling processes utilize a slurry typically consisting of clay and feldspars, or a frit (premelted glass) and water. This mixture is applied by brushing, spraying, or dipping and is subsequently heated to either remelt the glass or react and melt the glass-forming materials. These methods are obviously quite suitable for the preparation of thick films; however, due to the problems of wetting and adherence, it is virtually impossible to obtain thin, continuous films. When the aqueous suspension is applied to a substrate, there are no mechanisms by which it can chemically react with the substrate to promote adherence and wetting. Therefore, techniques which rely on mechanical adherence (such as picking metal surfaces to cause roughening) or which require high temperatures (i.e., application of dry enamel powders to very hot substrates or flame spraying) have been developed to overcome the adherence and wetting problems. However, multiple coatings are often still required to ensure continuity of the coating. Glazing and enameling processes, therefore, result in thick inflexible coatings which often require high processing temperatures.
In a somewhat related method, U.S. Pat. No. 3,212,929 requires that a glass film be deposited on a substrate using an organic solution containing powdered glass. The method is disadvantageous, inter alia, because it involves many steps, including a centrifugation operation.
Chemical vapor deposition can be used to produce glass films. However, the technique is expensive and, due to low deposition rates, is not particularly well-suited to the continuous production of, for example, glass coated wire. Also, the deposited film is porous and must be reheated to cause densification.
Immersion of substrates in molten glass or drawing them through molten glass does not involve satisfactorily low temperatures and, due to the relatively high viscosity of glass at all but the very highest temperatures, film thicknesses are generally high (&gt;10 .mu.m) resulting in rather inflexible films.
A less common method of preparing thin glass films is the sol-gel process disclosed, for example, in Brinker et al, "Sol-Gel Derived Antireflection Coatings for Silicon," Solar Energy Matls. 5(1981) 159-172; and Brinker et al, "Comparisons of Sol-Gel Derived Thin Films with Monoliths in a Multicomponent Silicate Glass System," Thin Solid Films 77(1981) 141-148, whose entire disclosures are incorporated by reference herein. In this process metal alkoxides of network forming cations, e.g., Si, Al, B, Ti, etc. are used as glass precursors. In alcoholic solutions, these alkoxides are partially hydrolyzed and then polymerized to form a glass-like network linked by bridging oxygen atoms. Dilute solutions (2-5 equivalent wt. % oxides) can be applied to metal, glass, and ceramic substrates by dipping, spinning, and spraying operations. When applied to a substrate, the partially hydrolyzed glass-like polymers react chemically with the surface and thus cause complete wetting. This is represented below where a silica-like polymer reacts with the hydroxylated monolayer of a metal, M, to produce direct M--O--Si bonds: ##STR1## As a result, excellent adherence is achieved. The microporous silica-like polymer film can then be converted to a dense glass film by relatively low-temperature heat treatments, i.e., heat treatments at temperatures much less than the glass softening point as shown, e.g., in Brinker et al, "Conversion of Monolithic Gels to Glasses in a Multicomponent Silicate Glass System," J. Mat. Sci. 16(1981) 1980-1988, whose entire disclosure is incorporated by reference herein.
This sol-gel process has the disadvantage that it can produce only very thin films by a single dipping step (generally less than 0.5 .mu.m). Only by repeated dippings can thicker films be produced; however, the rate of thickness buildup can be very slow and heat treatments between each successive coating are often required.